The Duet is calculating the Max Temp of the hotend. In several hotend designs, the thermistor is not mechanically attached to the hotend. If the thermistor separated from the heat block, the result could be a runaway hotend where temps get really hot. Our current kits include the newest E3D hotend where the thermistor is retained with a set screw. This minimizes the risk of the thermistor becoming separated. Regardless... we DO NOT recommend unattended printing.

The E3D hotend and Duet controller are the "Best in Class" components. That is why we choose them. The kit was not tested with lower current. Please post results if you dial back the power. Regardless... we DO NOT recommend unattended printing.

It looks like the best way to dial things back would be to decrease the PWM value. This would help in the case of your thermistor coming undone, but probably wouldn't help with a worst-case-scenerio thermal runaway situation where your heater was stuck in the 'on' position, not PWMing at all.

I'm having trouble digging up flash points of various 3D printing materials though; Makerbot puts their ABS's flash point at 404 degrees C, so it might make sense to dial down the PWM so that there's not a risk of going over 400 in the worst case scenario.